The invention relates to a method of sintering shaped bodies of silicon nitride (Si.sub.3 N.sub.4). More particularly, the invention provides such a method which is performed without high pressure or the addition of foreign substances.
For about ten years there has been great interest in silicon nitride, since it is a material of great potential for making articles exposed to high temperatures. Silicon nitride is characterized by a high decomposition temperature and good resistance to thermal shock. Furthermore, it is resistant to oxidation and is generally suitable for use in corrosive environments.
In spite of these outstanding properties, silicon nitride has not been used technically to any great extent, since Si.sub.3 N.sub.4 it is difficult to fabricate. Hitherto it has been possible to prepare silicon nitride ceramics of satisfactory mechanical properties only by hot pressing. This method, however, is not suitable for economical mass production; furthermore, the material can be shaped to only a limited extent. In addition, it is known to prepare "reaction-sintered" silicon nitride objects, but their mechanical properties leave much to be desired, and they are not comparable to the material sintered under pressure (not pressed).
It is known that in both of the methods mentioned above, the use of sintering aids is necessary. Aluminum oxide, magnesium oxide, zirconium oxide and yttrium oxide have proven most effective for this purpose. These metal oxides are added to the silicon nitride and the compacts prepared therefrom are then sintered. The addition of these sintering aids does permit sintered silicon nitride articles to be made, but at the same time it considerably impairs the very desirable properties otherwise inherent in silicon nitride.